[PATCH v2] mm: mark async iocb read as NOWAIT once some data has been, copied

[Jens Axboe <axboe@kernel.dk>]

Once we've copied some data for an iocb that is marked with IOCB_WAITQ,
we should no longer attempt to async lock a new page. Instead make sure
we return the copied amount, and let the caller retry, instead of
returning -EIOCBQUEUED for a new page.

This should only be possible with read-ahead disabled on the below
device, and multiple threads racing on the same file. Haven't been able
to reproduce on anything else.

Cc: stable@vger.kernel.org # v5.9
Fixes: 1a0a7853b901 ("mm: support async buffered reads in generic_file_buffered_read()")
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

V2
- Place the NOWAIT marker when we increment 'written' (Willy)

 mm/filemap.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1a6beaf69f49..508048bfd2a5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2329,6 +2329,15 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 		put_page(page);
 		written += ret;
+
+		/*
+		 * If we've already successfully copied some data, then we
+		 * can no longer safely return -EIOCBQUEUED. Hence mark
+		 * an async read NOWAIT at that point.
+		 */
+		if (iocb->ki_flags & IOCB_WAITQ)
+			iocb->ki_flags |= IOCB_NOWAIT;
+
 		if (!iov_iter_count(iter))
 			goto out;
 		if (ret < nr) {
-- 
2.28.0

-- 
Jens Axboe